Plaid Cymru’s group leader on the council has criticised the Labour-led authority’s handling of the project, which was carried out by construction firm Walters.

“Some of the reasons quoted for the vast increase could be put down to a lack of foresight when the contract was first awarded,” said Councillor Colin Mann.

“The huge chaos caused by the work shouldn’t have come as a surprise, if proper traffic counts had been taken.

“Single lane travel did have a massive impact on travel times, particularly for commuters travelling to and from work.

“Businesses situated locally have quoted massive losses in takings which could seriously threaten their future viability.”

Figures obtained by Plaid show the original costs before tender included pre-construction design fees, surveys, ecological mitigation as well as construction costs and post scheme liabilities.

Cllr Mann added: “This scheme is important in helping reduce congestion for drivers’ travelling to and from the Rhymney Valley. However, in anyone’s language, this represents a massive increase in costs.”

The project also received significantly more grant funding from the Welsh Government than originally anticipated.

But the council is expecting the final cost for the construction contract with Walters to be around £5.5 million.

A council spokeswoman said: “The overall estimate has undoubtedly increased prior to tender, with much of the additional cost arising from [Walters] revising their method of working on the southbound Llanbradach arm.

“These alterations were made after listening to concerns from the public, local councillors and businesses about disruption they were experiencing with the single lane operation.

The spokeswoman continued: “The effects of moving from single lane working and unforeseen service diversions, two significant snowfalls and other eventualities beyond our control had a significant impact on the works programme.

“Despite this, the council and contractor worked extremely hard to mitigate any extension to the works programme, with the project being delivered only a few weeks over the original scheduled 12 months, and more importantly, prior to the busy pre-Christmas trading period which is crucial for local businesses.”